Darth Vader never returned to the light. He just wanted to save his son, Luke. However, in later years, Luke Skywalker does go to the dark side to learn the energy that it possesses, and he later comes back to the light. This is after all the movies, when Leia is a jedi and all the main characters have kids. Hopefully the next trilogy can explain this.

I believe they meant that he 'fully' fell to the dark side, became a sith lord, and returned to the light and continued as on as a jedi; most other redemptions I can think of involve the fallen jedi redeeming themselves before they die (Vader) or the jedi briefly succumbing to the dark side, but returning to the light later (like Juhani, also from KOTOR)

Maybe...but I would still chalk it up to being him getting to the light side. He killed the emperor and saved the one who would train the new Jedi order (Luke). But now that you explain it like that, I might be willing to at least say you're not wrong.